In one aspect, the present invention relates to a process for producing a blend of polyalkylene terephthalate and melt processable wholly aromatic polyester. In another aspect, the present invention relates to an improved blend of wholly aromatic polyester and polyalkylene terephthalate. The characteristics and properties of such a blend differ significantly from those expected in light of observations of the properties of ordinary mixtures and of most polymeric blends.
When a blend or mixture is prepared from two or more ordinary, non-polymeric materials, a random distribution of the molecules of the components is obtained. This random distribution provides complete mixing without the formation of groups or clusters of the molecules of any one component. Such a mixture is expected to follow the "Rule of Mixtures". The Rule of Mixtures predicts the numerical values of properties, such as tensile and flexural strengths and tensile and flexural moduli, of a blend to be the weighted average of the numerical values of the properties of the components.
A discussion of the Rule of Mixtures can be found in the book Predicting the Properties of Mixtures: Mixture Rules in Science and Engineering, by Lawrence E. Nielsen, Marcel Dekker Inc. (New York).
Further information with regard to the Rule of Mixtures can be found on pages 395, 436, 465, 492, and 500 of Volume 2 of Mechanical Properties of Polymers and Composites, by Lawrence E. Nielsen, Marcel Dekker Inc. (New York: 1974). As stated therein, mixtures of a polymer matrix with a fibrous reinforcing agent, a ribbon-shaped filler, or a rod-shaped filler are known to follow the Rule of Mixtures. The above-cited reference further discloses that mixtures of phase inverted isotropic interpenetrating polymer networks, such as a phase inverted network of polystyrene and polybutadiene, are also known to follow the Rule of Mixtures.
Mixtures of most chemically distinct polymeric materials have been found to deviate from the behavior of ordinary mixtures as characterized by the Rule of Mixtures. The sheer size of polymeric chains restricts mixing of the components and leads to the formation of domains or clusters of molecules of the individual components. Thus, it can be said that most chemically distinct polymeric materials tend to be incompatible in mixtures, exhibiting a tendency to separate into phases. There exists a boundary between the domains of the component polymers, and articles made from such a blend would be expected to exhibit failure at the boundary when placed under stress. In general, then, the mechanical properties of the product are commonly reduced rather than enhanced. Specific properties which may be thus affected include tensile strength, tensile modulus, flexural strength, flexural modulus, and impact strength.
Some polymeric materials exhibit an ordered structure in at least some regions of the polymer. This order can exist in one, two, or three dimensions. The inclusion in blends of polymeric materials exhibiting an ordered structure leads to an increased tendency of the blends to separate into phases. This is due to the fact that the order found in certain regions of the polymer causes a fairly sharp boundary between the domains of the molecules of the component polymers. Thus, blends including such polymers would be expected to exhibit a significant reduction in mechanical properties. Accordingly, there has been little impetus to form such blends, particularly for use in applications where mechanical properties are of importance.
U.S. Pat. No. 4,228,218 discloses a polymer composition comprising 20 percent or less, based upon the total weight of polymeric material, of a first rigid polymeric material with the balance being a second polymeric material composed substantially of flexible molecular chains. The first polymeric material is dispersed in the second polymeric material in a microscopic region of 1 .mu.m. or less. Foreign counterparts of this application include Japan Pat. No. 54065747, French Pat. No. 2407956, West German Pat. No. 2847782, and British Pat. No. 2,008,598A.
Commonly assigned U.S. application Ser. No. 158,547, filed June 11, 1980, entitled "Blend of Polyalkylene Terephthalate and Wholly Aromatic Polyester," discloses a blend which comprises approximately 5 to approximately 75 percent by weight of a polyalkylene terephthalate and approximately 25 to approximately 95 percent by weight of a melt processable wholly aromatic polyester. However, the blend there exemplified utilized wholly aromatic polyester of relatively low molecular weight which had not been subjected to post-polymerization. Moreover, there is no indication that the blend contained chemical interaction between the blend components as is required in the process and product of the present invention.
Commonly assigned U.S. application Ser. No. 330,998, filed Dec. 15, 1981, entitled "Blend of Polyalkylene Terephthalate and High Molecular Weight Wholly Aromatic Polyester and Process for Producing the Same" (Inventors: David E. Cincotta and Frank M. Berardinelli), discloses a blend which comprises approximately 5 to 15 percent by weight of a polyalkylene terephthalate and 85 to 95 percent by weight of a wholly aromatic polyester, the wholly aromatic polyester having been subjected prior to blending to post-polymerization so as to increase the inherent viscosity thereof by at least approximately 50 percent. The compositional ranges of the blend and the sequence of steps of the process there disclosed differ significantly from those of the blend and process of the present invention.
See also European Patent Application No. 30,417, published June 17, 1981 and entitled "Compositions of Melt-Processable Polymers Having Improved Processability and Method of Processing."
At the time the present invention was made, it was known to increase the molecular weight, as indicated by an increase in inherent viscosity, of wholly aromatic polyester by subjecting the wholly aromatic polyester to a post-polymerization process, such as solid state polymerization, as illustrated in Example II of commonly assigned U.S. Pat. No. 4,161,470. It was further known that high molecular weight wholly aromatic polyester often exhibited better properties than low molecular weight wholly aromatic polyester. However, at the time the present invention was made it was not obviously apparent that higher molecular weight polyesters would be particularly desirable for blending with polyalkylene terephthalates, since the melting temperature of the wholly aromatic polyester tends to increase with increasing molecular weight and the increased molecular weight could possibly impede the desired admixture of the polymer components or otherwise impede processing stability thereby resulting in poorer blend properties.
It is therefore an object of the present invention to provide a process for producing a blend of polyalkylene terephthalate and wholly aromatic polyester which exhibits satisfactory mechanical properties, such as tensile strength, tensile modulus, flexural strength, flexural modulus, and impact strength.
It is also an object of the present invention to provide a process for producing a blend of polyalkylene terephthalate and wholly aromatic polyester which exhibits no significant reduction in mechanical properties as compared to the weighted average of the mechanical properties of the individual components.
It is also an object of the present invention to provide a blend of polyalkylene terephthalate and wholly aromatic polyester which exhibits satisfactory mechanical properties, such as tensile strength, tensile modulus, flexural strength, flexural modulus, and impact strength.
It is also an object of the present invention to provide a blend of polyalkylene terephthalate and wholly aromatic polyester which exhibits no significant reduction in mechanical properties as compared to the weighted average of the mechanical properties of the individual components.
It is also an object of the present invention to provide a blend of polyalkylene terephthalate and wholly aromatic polyester which exhibits a high degree of anisotropy in the melt.
It is a further object of the present invention to provide an improved polymer blend comprising a wholly aromatic polyester which can be more economically produced without a substantial reduction in mechanical properties.
These and other objects and advantages will be apparent to those skilled in the art from the following detailed description and appended claims.